1. Field of the Invention
This invention relates to the use of a scanning localized evaporation methodology for processing of multilayer, patterned electronic and photonic devices, such as transistors, sublimable organic light-emitting diodes (OLEDs), photonic band gap structures, and integrated circuits/systems. One example is the fabrication of displays using OLEDs for applications ranging from computer monitors to personal digital assistant (PDAs).
2. Description of the Related Art
Introduction:
A typical display screen comprises of a regular array of color pixels, each of which can be electrically selected to emit red, green and blue colored light, resulting in a desired shade and brightness. Each pixel is comprised of three devices, a red, a green, and a blue light-emitting element. Liquid crystal displays make use of color filters whose transmissions are selectively blocked to produce red, green, blue pixels. By choosing a single device or a combination of the three, a wide range of colors can be produced. Pixels are produced via patterning of one or more thin film layers deposited on a suitable substrate.
In the traditional electronics industry, lithographic and etching techniques are used to selectively remove portions of the blanket films, leaving behind the desired pattern.[1] Recently, display,[2] electronic[3, 4] and photonic devices,[5, 6] using organic semiconducting materials (of both low and high molecular weight), have shown certain advantages over traditional inorganic materials. These organic materials, due to the chemical sensitivity to both solvents and lithographic procedures, however, require new fabrication methodologies for both deposition and patterning.[7, 8]
One of the techniques typically employed is evaporation of these materials through shadow masks.[9] This process is limited to relatively large feature sizes. For finer features/pixel sizes, smaller pre-deposited patterns of inert resist materials are employed to serve as shadow masks.[10] In another methodology, films are deposited on substrates on which have been produced three-dimensional pyramidal structures with triangular bases, each face corresponding to the one of the three primary colors.[11] These techniques, however, have a number of limitations, such as cost associated with background patterning and multi-step batch processing. Until recently, due both to their high purity and to the ease of producing multilayer device structures,[12-15] sublimable organics have been in the forefront of display and transistor development. However, the fabrication of fill color displays through the adaptation of ink-jet printing for polymeric semiconductor has provided an alternate technology.[16-18] This technology requires the use of specialized substrates. These substrates must have indentations, exhibiting controlled wetting characteristics, which serve as micro-containers or wells for localizing the deposited polymeric solution, prior to drying.
Forrest et al. [19] reported a systematic and quantitative study on the design and limitations of OLED-based flat panel displays (FPDs). Among the various addressing schemes used in electronic displays,[20] direct and matrix addressing are suitable for OLEDs. The direct addressing scheme, where each pixel is connected to an individual driver, can only be used for discrete indicators and simple alphanumeric displays with few characters. In a matrix-addressed display, pixels are organized in rows and columns, and each pixel is electrically connected between one row lead and one column lead. The addressing schemes, where active electronic components are added to the pixels, are called active-matrix addressing;[21] while those without extra active components are termed passive-matrix addressing.[22]
FIG. 1 shows typical passive (a) and active matrix (b) architectures for full color organic light emitting diode (OLED) flat panel displays (FPDs). The red (8), green (9) and blue (10) electroluminescent (EL) materials 2 shown separately in FIG. 1c and combined as layer 2 in FIG. 1a and 1b are typically sandwiched between transparent conducting indium tin oxide (ITO) and metallic cathode electrodes to produce separate red, green and blue light emitting areas, which constitute a full color pixel. The major difference between the passive matrix architecture and the active matrix architecture is in the patterns of the electrodes. For the passive architecture, the cathode 1 and the anode 3 consist of line structures that intersect perpendicularly to define the elements of the full color pixel, any one of which can be activated by powering the row and column defining that element, whether it be red, green or blue. For the active matrix architecture, the emitting materials 2 are sandwiched between the ITO pads 5 and the common cathode 4.[23] Not shown in FIG. 1b are the addressable transistors, which connect the individual ITO pads 5 of each pixel element of the full-color display.
A typical OLED construction starts with ITO patterns on the substrate 13, a common Anode Modifying Layer (AML) 12 (ie. copper phtalocyanine), Hole Transport Layer (HTL) 11 (i.e. N,N′-Bis(naphthalen-1-yl)-N,N′-bis(phenyl) red emitting layer 8 (i.e. 4% of 2,3,7,8,12,13,17,18-octaethyl-21H,23H-porphine platinum(II) (PtOEP) doped within aluminum (III) 8-hydroxyquinoline (Alq3)), green emitting layer 9 (i.e. Alq3 or 0.8% N,N-dimethyl quinacridone doped within Alq3), and blue emitting layer 10 (i.e. lithium tetra-(8-hydroxy-quinolinato) boron (LiBq4)), Electron Transport Layer (ETL) 7 (i.e. Alq3 or bathocuproine (BCP)), Cathode Modifying Layer (CML) 6 (i.e. cesium floride or lithium floride), and Cathode Layers 1 or 4 (i.e. aluminum or magnesium). In reality, some of these layers might need to be different for each art.
FIG. 2 describes typical thin film vacuum evaporation method that relies on resistively heated boats or filaments 15 that deposit thin films on substrate 13 through a mask 14. The deposition source could also be a laser or e-beam heated target 15. Alternately, sputtering, plasma or glow discharge methods can be employed. All of the above deposition techniques generally require a significant distance to be maintained between source 15 and substrate 13 to obtain the desired film thickness uniformity.